Add tests for the web build of the Pylance extension

[luabud]

https://github.com/microsoft/vscode-python/issues/19115

https://code.visualstudio.com/api/extension-guides/web-extensions#web-extension-tests

https://github.com/microsoft/vscode-extension-samples

https://github.com/microsoft/vscode-jupyter/commit/cd787d290efe0ee3bebad98747751da02fcc4c58